As a prior art that employs a field-effect transistor, in which a memory film for accumulating electric charges in its gate insulation film, as a memory element, there can be enumerated a flash memory. The flash memory has a conductor film, which is called a floating gate in its insulation film located between its control gate and its channel region. The quantity of charges in the floating gate is changed by injecting or discharging electrons from the channel region to the floating gate by FN (Fowler-Nordheim) tunneling, and the quantity of charges is retained as storage information. In reading the stored information, the quantity of charges in the floating gate can be detected as a difference in the threshold of the field-effect transistor.
However, according to the above-mentioned prior art, there has been a problem that the operating voltages have been high. As an example of operation of the flash memory, for example, a voltage of −8 V is applied to the selected word line and a voltage of 6 V is applied to the selected bit line during write. A voltage of 10 V is applied to the selected word line and a voltage of −8 V is applied to the bit line during erase. Since the operating voltages have been high as described above, the consumption power during write and erase have been large, and this has hindered a reduction in the consumption power. Moreover, since a high electrical field is applied to the gate insulation film, element deterioration has been a problem.